In recent years, secondary cells have been used across a very wide range of files and industries including electric automobiles and mobile telephones, and in accompaniment with the applications thereof, there are increasing performance-related demands for higher energy densities, higher output densities, faster charging/discharging, etc.
Various studies and development pertaining to various materials have been carried out in order to satisfy these demands, an example thereof being the development of an electroconductive agent that is optimal for a secondary cell.
Since electrode active materials themselves typically have low electroconductivity, an electroconductive agent is used during electrode production. The electroconductive agent enhances electroconductivity in an electrode by forming an electroconductive path between individual electrode active material particles or by covering the surface of the electrode active material to enhance the electric current collection effect, and has the effect of improving the performance of a secondary cell, which is the final product.
Acetylene black has been widely used as an electroconductive agent in the prior art. Acetylene black is dispersed and present in the electrode active material during use and forms a path between particles, but at present, since the particle diameter is relatively large, it is difficult to uniformly cover the surface of the electrode active material, the electric current collection effect is therefore low because the contact locations with the electrode active material are limited, and sufficient electroconductive effect cannot be obtained.
Acetylene black has few surface functional groups for reasons related to manufacturing methods, and does not readily disperse in aqueous solvents. Since this is a negative factor in relation to the electroconductive effect, there is a need for an electroconductive agent that has better performance.
In response to this problem, investigations are being made on electroconductive carbon black, in which the carbon black mainly manufactured by the furnace method has defined physical properties. Patent Document 1, for example, discloses a technique for using carbon black to manufacture an electrode active material for a lithium secondary cell, the carbon black having particles with a small diameter in which the nitrogen adsorption specific surface area (N2SA) is 200 g/m2 or higher.
Patent Document 2 discloses a technique for covering the surface of the electrode active material by a combined used of two types of carbon black having different specific surface areas.
In addition, there are also inventions that specify the DBP absorption amount, which is an index of the aggregation size as the minimum unit of carbon black, and these conventional carbon blacks exhibit a constant effect in terms of covering the surface of the electrode active material. However, there is room for improvement in that there has been no research that focuses on the shape of carbon black optimal for covering.
In other words, the definition of carbon black for electrode applications is specified by the specific surface area of the carbon black and/or the DBP absorption amount. As far as the present applicants are aware, there is no example of the shape of a carbon black aggregate being observed from the viewpoint of the level of development of a carbon black aggregate as evaluated using the diameter of the primary particles constituting the carbon black aggregate and the number of these primary particles, and using these characteristics to specify the shape of an optimal carbon black aggregate in terms of covering the electrode active material of a secondary cell.